The present invention is directed to a tunable semiconductor laser on a semi-insulating substrate having an active layer and a tuning layer, one arranged above the other.
It is necessary for numerous applications of semiconductor lasers to vary the wavelength of the emitted light as rapidly and steadily as possible. One example of this is optical heterodyne reception wherein the semiconductor laser serves as a local oscillator in the receiver for demodulation of the signal. European Patent Application EP 0 360 011 discloses a tunable DFB laser wherein an active layer and a tuning layer are vertically arranged relative to one another and separate current injection is possible via an intermediate layer arranged therebetween and via lateral regions. This laser is particularly suited (tuneable twin guide-distributed feedback laser diode, also referred to as a TTG-DFB laser diode) for the aforementioned purpose because of its broad, continuously variable tunability.
In prior art embodiments, this laser diode is manufactured on a p-doped substrate that represents the electrical lead to the active layer or to the tuning layer depending on which of these layers is located on a side of the intermediate layer facing toward the substrate. As a result, the substrate cannot be connected to the common electrical ground of the further components, but must be electrically insulated from the common sub-carrier. A TTG-DFB laser on an electrically insulating substrate that is simpler to manufacture and more mechanically stable is disclosed in German Patent Application P 40 37 896. The type of laser required for this purpose whose electrical contacts must all lie on one side of the laser layers for a semi-insulating substrate is also defined by a monolithically integrated, passive optical waveguide. The arrangement of the three required contacts in the laser diode of this patent application is facilitated in that adequate space for the bond area of at least one contact is available over the integrated waveguide. One disadvantage of this arrangement is the nonplanar surface of the laser diode that has steps several micrometers in height. It makes the manufacture of the contacts more difficult and diminishes the mechanical stability of this arrangement. The passive waveguide that enables the adequate bond areas is only of use in the monolithic integration of the laser diode.